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%I #31 Oct 19 2023 22:32:31
%S 0,1,6,9,10,12,13,23,61,194,233,549,765,973,1061,1186,1853,3713,6789,
%T 7254,7765,9025,10855,23640,31440,31839,32287,120342,132148
%N Numbers n such that 9*10^n-7 is prime.
%C Also numbers n such that 8*10^n + 9*R_n - 6 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
%C a(30) > 3*10^5. _Robert Price_, Oct 19 2023
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/8/89993.htm#prime">Prime numbers of the form 899...993</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A101079(n-1) + 1 for n>1.
%e For n=0, 9*10^n-7=2 which is prime, so 0 is in the sequence.
%t Do[ If[ PrimeQ[9*10^n - 7], Print[n]], {n, 0, 10000}]
%o (Magma) [n: n in [0..300] | IsPrime(9*10^n-7)]; // _Vincenzo Librandi_, Sep 05 2015
%o (PARI) is(n)=ispseudoprime(9*10^n-7) \\ _Charles R Greathouse IV_, Jun 12 2017
%Y Cf. A002275, A101079.
%K more,nonn
%O 1,3
%A _Robert G. Wilson v_, Jan 19 2005
%E One more term from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
%E a(23)-a(26) from Kamada data by _Robert Price_, Dec 14 2010
%E Inserted a(1)=0 by _Robert Price_, Sep 04 2015
%E a(28)-a(29) from _Robert Price_, Sep 04 2015
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